For the action of a conventional electrical bell, wire is a must, and generally, A.C. power is used. It does not matter whether a conduit is needed outside or inside the wall, an electrician is needed to design and a install it, which requires labor, material and time. Sometimes, it can even ruin a door or wall and detracts from the exterior appearance of the building. If an A.C. power source is used, any power failure is inevitable and accidental leakage shock could also be a hazard. Also, a conventional electric bell merely serves the normal function of notifying the occupant that someone is at the door and that he must answer.